1. Field of the Invention
The present invention relates to a manufacturing method of a semiconductor device having Si/GeSi superlattice structure selectively formed on the silicon substrate, which is useful as an optoeletric integrated circuit (OEIC) receiver.
2. Description of the Related Art
The first example of a prior art relative to a photodetector is, as shown in FIG. 1, made by the following method: on the P.sup.+ silicon substrate 18, a Si/Ge.sub.x Si.sub.1-x superlattice layer 7 having superlattice structure of alternate layer of approximately 30 .ANG. of Ge.sub.0.8 Si.sub.0.4 and approximately 290 .ANG. of Si formed by MBE (molecular beam epitaxy) method to the total thickness of 6000 .ANG..
Next, on said Si/Ge.sub.x Si.sub.1-x superlattice layer 7, a P type silicon layer 6 is formed by MBE method and on the silicon layer 6, an N.sup.+ type silicon layer 19 is formed. Then, as shown in the FIG.1, the N.sup.+ type silicon layer 19 is converted to a mesa shape by etching. A silicon oxide film 9 is deposited on the silicon layer 6 by CVD method and an opening 9a is made on the oxide film 9 by selectively removing a part of the silicon oxide film 9 above the silicon layer 19. Then, aluminum electrodes 11 are attached to the base and top faces of the substrate (IEEE ELECTRON DEVICE LETTERS VOL. EDL-7, No. 5, 1986 pp 330-332).
As the second example of a prior art, a manufacturing method of a superlattice element as shown in Japanese Provisional Publication (hereinafter referred as TOKKAI) HEI 3-53567 is explained in order of its process. As shown in FIG.2A, a 2 .mu.m thick cladding layer 21 made of Al.sub.x Ga.sub.1-x As is formed on a GaAs substrate 20 by MBE method, and then, a superlattice layer 22 of 640 .ANG. thickness consisting of 4 cycles of an 80 .ANG. GaAs layer and an 80 .ANG. Al.sub.x Ga.sub.1-x As is formed and a 1 .mu.m thick upper cladding layer 23 made of Al.sub.x Ga.sub.1-x As is formed.
Next, as shown in FIG.2B, a ridge structure 2 .mu.m wide and 12.mu.m high is formed by using photolithographic technique and RIE technique. Then, a silicon oxide layer 24 is formed on the surface of the ridge by plasma CVD method.
Then, as shown in FIG.2C, while the surface where a silicon oxide layer 24 is formed and another GaAs wafer (not shown) are put together, heat treatment is performed under hydrogen atmosphere to have partially mixed crystallization take place in the superlattice layer 22 from the sides contacting silicon oxide layer 24 in transverse direction so as to have a superlattice structure retained in the central parts only of the ridge structure and a mixed crystal layer 25 on the both sides of the superlattice structure.
In this example of prior art, although the sides are protected by the ridge structure and with the silicon oxide layers 24, the side walls are made to a mixed crystal layer 25 so that any adverse effects due to damages on the treated surfaces can be avoided.
Another example of a combination of an OEIC receiver having a Ge.sub.1-x Si.sub.x mixed crystal layer and a device is the one described in TOKKAI SHO 63-122285. A sectional view of the MOS type image sensor is shown in FIG.3. A buffer layer 26 which comprises gradient ratio of Si to Ge.sub.0.85 Si.sub.0.15 mixed crystal in the direction of the thickness is formed on the Si substrate 1 by, for example, MBE method. Then, an N type Ge.sub.0.85 Si.sub.0.15 layer 27 is formed by MBE method, a P.sup.+ drain region 29 and a P.sup.+ source region 30 are formed on the surface of the substrate 1, a silicon oxide film 28, a drain electrode 31 and a gate electrode 32 are formed and then an OEIC receiver element and a MOS type image sensor are simultaneously formed on the Si substrate.
There are, however, weak points in all said prior technologies, which will be explained below.
The photodetector in the first example is composed of a so-called superlattice structure with a multiple layer structure of Si layers and Ge.sub.1-x Si.sub.x layers in order to enhance optical receiving efficiency.
Although this is an attempt to improve the efficiency covering the small optical absorption coefficient of silicon, there is a limitation in this attempt because it is difficult to make the thickness of the layers of the superlattice any more than 6000 .ANG. due to a problem of crystallization. Therefore, in order to improve the optical receiving efficiency, it is necessary to minimize the loss at the end face of the regions other than the thickness of the light absorption layer. In this first example, the side walls of the Si/Ge.sub.1-x Si.sub.x superlattice is either as they were cut out or in a structure covered with a silicon oxide film in a mesa structure. Therefore, it is difficult to improve the absorption efficiency because of the loss of the incident light from the transverse direction at the side walls of the above mentioned structure. Although this structure enables the distance for absorption to be adjusted by the transverse distance of the Si/Ge.sub.1-x Si.sub.x superlattice when the incident angle of the light is transverse to the substrate, it is difficult to have the light precisely incident upon the Si/Ge.sub.1-x Si.sub.x end face by fixing optical fiber which is the source of the incident light.
There are other disadvantages attending the first two examples of prior art discussed above. Namely, if the end face of the superlattice is etched to form a mesa structure, the end faces are apt to be damaged. Also there is a reliability problem due to the energy level in the oxide film which covers the superlattice layers. In the prior art, a method to protect the etching side by mixed crystallization from the end face is adopted in order to solve the problem. While this method can be applied to a GaAs substrate, it cannot be applied to a Si substrate,
According to the third example prior art, of an element to receive incident light from the top surface is formed, which is combined with other elements. However, in this method, it is necessary to make the area of the light receiving part large in order to improve optical receiving efficiency. Also, because the Si/Ge.sub.x Si.sub.1-x superlattice layer is formed on the entire face, combination with other elements is difficult and adoption of a Si device is impossible. Therefore, it is difficult to incorporate a large scale integrated circuit because the ratio of the area occupied by the light receiving region of the chip becomes large due to the large area of the light receiving element. Furthermore, as discussed before, because a Si/GeSi layer is formed on the entire face of the substrate, simultaneous formation of a bipolar device is difficult in the case of the third example of prior art.